1. Field of the Invention
The invention relates to a level shift circuit, in particular, a level shift circuit using a differential amplification circuit.
2. Description of the Related Art
An optical pickup (a recording reproducing component) is used in a recording reproducing device that reproduces data recorded in an optical disk such as a DVD or a CD. A signal from the optical pickup is processed by a signal processing LSI. When the supply voltage of the optical pickup and the supply voltage of the signal processing LSI are different, it is necessary to shift the signal from the optical pickup by a level shift circuit and input it to the signal processing LSI.
When a signal from a 5V type optical pickup is to be processed by a 3.3V type signal processing LSI, the shifting amount by the level shift circuit is so small that it does not exceed the threshold of a normal transistor. For example, when the direct current level of the signal from the 5V type optical pickup is 2.1 V, the direct current level of the signal of the 3.3V type signal processing LSI is shifted to 1.65 V. The shifting amount in this case is 0.45 V.
Therefore, the level shift circuit is configured using a differential amplification circuit instead of using a source follower circuit. In this case, the signal from the optical pickup is applied to the non-inversion input terminal of the differential amplification circuit, and the reference voltage (e.g. 3.3 V/2=1.65 V) of the 3.3V type signal processing LSI is applied to the inversion input terminal thereof.
A relevant technology is described in Japanese Patent Application No. Hei 6-104751.
However, in the level shift circuit using the differential amplification circuit, the signal level inputted to the non-inversion input terminal of the differential amplification circuit depends on the variation of the supply voltage of the optical pickup, while the reference voltage inputted to the inversion input terminal of the differential amplification circuit depends on the variation of the supply voltage of the signal processing LSI. Therefore, when these supply voltages respectively change to the opposite sides, i.e., when one changes to the negative side and the other changes to the positive side, there occurs a problem that the direct current level of the output signal of the differential amplification circuit changes largely.
It means that a large offset occurs in the direct current level of the output signal of the differential amplification circuit (a shift from the predetermined direct current level) to cause the malfunction of the signal processing LSI.